1. Field of the Invention
The present invention relates to a method and device for computing computer-generated holograms (CGH), especially real-time or near real-time holograms, e.g. video holograms, which are made up of individually controllable hologram cells; each cell displays complex-valued data. Besides stills, real-time video holograms are of particular interest. Electro-holography aims at a realization of CGH in real-time. The electro-hologram display is effectively a Spatial Light Modulator (SLM) with controllable pixels reconstructing object points by spatial modulating an illuminating light. Throughout this specification, we will refer to real-time holograms as video holograms. For those skilled in the art, video holograms also cover Optically Addressable SLMs, Acousto-Optic light Modulators (AOM) or the like which do not exhibit separately arranged cells.
In contrast to classic holograms, which are stored photographically or in another suitable way in the form of interference patterns, video holograms exist as a result of a computation of discrete hologram data from sequences of a three-dimensional scene. During the computation process, the intermediate data is stored, for example, by electronic means, such as an electronic storage medium of a computer, graphics processor, graphics adapter or other hardware component. The 3D scene data can be generated in any way, e.g. by interference patterns or 3D conversion of 2D data.
2. Background Concepts
Spatial Light Modulators (SLMs) are devices for spatially controlling the complex-valued data, i.e. the magnitude and phase of the amplitude of each color component of light. The color can be encoded by being spatially or temporally multiplexed. The SLM may contain controllable hologram cells, each being separately addressed and controlled by a discrete value set of a hologram data. SLMs can also be continuous and not contain discrete cells. To achieve color encoding by spatial multiplexing in a cell based SLM, each pixel in a cell may comprise color sub-pixels, each sub-pixel displaying one of three or more primary colors. Depending on the kind of video hologram encoding used, further sub-pixels may be used for encoding each of the primary colors. For instance, a detour phase encoding, like the known Burckhardt encoding, needs an arrangement of three sub-pixels for each color component. Taking into account three color components, the number of sub pixels totals to nine for a hologram cell (i.e. there are three primary colors; there are three sub-pixels for each of these three primary colours, making nine sub-pixels in total. In contrast, the also known Lee encoding requires four sub pixels; and a two-phase encoding requires two sub pixels for each color in a hologram cell.
Each hologram cell is encoded by one discrete set of hologram data containing at least amplitude and phase information of a given color component; said data may be zero or have a standard value or may be arbitrarily chosen. The hologram data of a video hologram is continuously updated according to the scheme driving the SLM. Since the entire hologram is made up of thousands of cells, there are thousands of discrete sets of hologram data.
A hologram data set contains all the information necessary to encode one single video hologram cell as part of a time sequence to reconstruct a three-dimensional scene.
A dedicated driver uses the discrete hologram data sets to provide the specific control signals for controlling the corresponding sub-pixels of the SLM. The driver and the provision of control signals are specific to the type of the SLM used and is not the subject of this invention. Many kinds of SLMs, like transmissive or reflective liquid crystal displays, micro optical and electro mechanical micro systems or continuously optically addressed SLMs and acousto optic modulators can be used in combination with this invention.
The modulated light emerges from the hologram with the amplitude and phase appropriately controlled and propagates through the free space towards the observer in the form of a light wave front, to reconstruct a three-dimensional scene. Encoding the SLM with the hologram data set causes the wave field emitted from the display to reconstruct the three-dimensional scene as desired by creating interferences in the viewing space.
The present invention provides real-time or near real-time control data for each hologram cell for the required wave modulation by computing amplitude and/or phase for a given wavelength.
3. Description of Related Art
A common problem in reconstructing three-dimensional scenes is the low pixel resolution and low pixel count currently feasible with conventional SLMs. For reconstructing 20 inch wide SLMs available today a pixel pitch about 1 μm-would be required check meaning. Taking into account three sub pixels for encoding each of the three primary color components in a hologram cell, —more than 109 pixels would be necessary. This requires costly hardware and high computational speed for calculating the video hologram. Affordable real-time displays and devices with fast enough computational speed which meet these demands are currently not commercially available.
For computing video holograms it is not necessary for 3D scenes to have existed in reality. This enables reconstruction of virtual 3D scenes in various fields of applications, such as technology, entertainment and advertising, where moving three-dimensional scenes are synthesized and edited by computer.
Computer-generated video holograms can, for example, be reconstructed using a holographic display as described by the applicant in document WO 2004/044659, the contents of which are incorporated by reference. The viewer looks towards the display screen through at least one virtual observer window, which is greater than an eye pupil. The observer windows are located near the viewer's eyes and can be tracked to follow the viewer's position with the help of known position detection and tracking devices. The image plane of the light sources is the Fourier plane of the hologram. As the observer window is part of the Fourier plane of the hologram, it is on the image plane of the light source.
The observer windows can therefore preferably be limited to a size just a little larger than the size of the eye pupils. This greatly reduces the requirements on the pixel resolution and pixel count of the SLM and reduces the computational load. Consequently, the data transfer rate and the required computing power can be reduced and a light modulator matrix with low resolution can be used. One disadvantage of the encoding technique described in this application is that it is based on a computationally intensive operations performed on every single point in the object to be reconstructed.
Video holograms which are computed according to this invention can be reconstructed for example using pixel arrays of about 3 million pixels.
WO 03/025680 discloses a method for computing a video hologram with a restricted grayscale range for representation. A target hologram is divided into partial holograms and their individual reconstructions are used for iteratively computing optimized sub-holograms, thus reducing the required computing power. The iteration process is repeated until the sub-holograms with a small grayscale range can be composed to form a total hologram with an accordingly small grayscale range. In order to convert the serial processing into computational steps which can be carried out simultaneously, separate reconstructions of each sub-hologram are optimized independently of each other until the desired result is achieved for the total hologram. After having generated a target wave front for each data set, the sub-holograms are composed. However, although parallel processing when computing the optimized sub-holograms increases the processing speed, the required computing power is not reduced.
WO 00/34834 discloses a method for calculating three-dimensional scenes and for their real-time reconstruction from digital image data using LCDs. The image data describe a real or virtual three-dimensional scene by their intensity distribution in space. The main steps are: dividing a 3D scene into several parallel section layers (slicing) with respective section boundaries of the scene, computing a section hologram for each section layer, and sequentially reconstructing the computed section holograms using a light modulator matrix. For each section hologram, the given two-dimensional image defined by an intensity distribution is transformed into a two-dimensional intermediate image defined by a complex function. The resolution of the three-dimensional reconstruction is increased by way of over sampling over sampling the images. Then, a fictive diffraction image is computed for each scene section in a reference layer situated at a distance to the section layers, and the diffraction image is superimposed by a complex reference wave. This results in expressions of two-dimensional holograms in the form of interference patterns for the reference layer, said patterns providing discrete control values for a driver to encode the light modulator matrix. The light modulator matrix is situated in the reference layer with this prior art solution.
The diffraction images of the section layers are computed by multiplying the complex pixel amplitude values and the mathematical expression of a spherical wave according to the distance between this section layer and reference layer, and integration over all pixels of the scene section (slice). This integral is interpreted as a convolution integral and evaluated by computing the product of the Fourier transform of the factors and subsequent back-transformation.
A disadvantage is that real-time sequential reconstruction of each section layer requires extremely fast computing means and a light modulator matrix which is capable of reconstructing several hundreds of section holograms per second. Moreover, the three-dimensional scene is reconstructed behind the reference layer. This means that a viewer sees the 3D scene behind the light modulator matrix, or inside the hologram display.
Because a proper reconstruction of the depth of a scene involves more than 100 section layers, this solution requires an extremely high refresh rate of the display screen. A satisfactory and natural real-time reconstruction of moving three-dimensional scenes using known computing and displaying means is not expected due to the low speeds and the fact that the reconstruction is restricted to the interior of the hologram display.